A joint within the human body forms a juncture between two or more bones or other skeletal parts. The ankle, hip, knee, shoulder, elbow and wrist are just a few examples of the multitude of joints found within the body. As should be apparent from the above list of examples of joints, many of the joints permit relative motion between the bones. For example, the motion of sliding, gliding, hinge or ball and socket movements may be had by a joint. For example, the ankle permits a hinge movement, the knee allows for a combination of gliding and hinge movements and the shoulder and hip permit movement through a ball and socket arrangement.
The joints in the body are stressed or can be damaged in a variety of ways. For example, the gradual wear and tear is imposed on the joints through the continuous use of a joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces direct to the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in direct contact with each other. In contrast, in normal use, a trauma to a joint, such as the delivery of a large force, from an accident for, example, an automobile accident, may cause considerable damage to the bones, the cartilage or to other connective tissue such as tendons or ligaments.
Arthropathy, a term referring to a disease of the joint, is another way wherein a joint may become damaged. Perhaps the best-known joint disease is arthritis, which is generally referred to as a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity.
There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within a joint. Another type of arthritis is osteonecrosis, which is caused by the death of a part of the bone due to loss of blood supply. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis destroy cartilage and are associated with the inflammation of the joint lining.
The hip joint is one of the joints that is commonly afflicted with arthropathy. The hip joint is a ball and socket joint that joins the femur or thighbone with the pelvis. The pelvis has a semispherical socket called the acetabulum for receiving a ball socket head in the femur. Both the head of the femur and the acetabulum are coated with cartilage for allowing the femur to move easily within the pelvis. Other joints commonly afflicted with arthropathy include the spine, knee, shoulder, carpals, metacarpals, and phalanges of the hand. Arthroplasty as opposed to arthropathy commonly refers to the making of an artificial joint. In severe cases of arthritis or other forms of arthropathy, such as when pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint within an artificial joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question, but in general involves replacing a terminal portion of an afflicted bone with a prosthetic implant and inserting a member to serve as a substitute for the cartilage.
The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and the cartilage substitute members chosen to provide lubrication to the joint and to absorb some of the compressive forces. Suitable material for the implant include metals, and composite materials such as titanium, cobalt chromium, stainless steel, ceramic and suitable materials for cartilage substitutes include polyethylene. A cement may also be used to secure the prosthetic implant to the host bone.
A total hip replacement, for example, involves removing the ball shaped head of the femur and inserting a stem implant into the center of the bone, which is referred to as the medullary canal, or marrow of the bone. The stem implant may be cemented into the medullary canal or may have a porous coated surface for allowing the bone to heal directly to the implant. The stem implant has a neck and a ball shaped head, which are intended to perform the same functions as a healthy femur's neck and a ball shaped head.
A cup or shell may be positioned directly into the acetabulum. The cup or shell may include a porous coating for promoting bony in-growth to secure the shell to the acetabulum. Alternatively or in addition, the shell may include an opening or a plurality of openings for receiving bone screws to assist in the attachment of the shell to the acetabulum. The cup may be made of a metal, for example, cobalt chromium, stainless steel, or titanium. Alternatively, the cup may be made of a ceramic or of a polyethylene. In some embodiments, the cup directly engages the head. In other embodiments, a liner of some sort is inserted into the cup to articulate against the head. The liner may be made of metal, ceramic, or polyethylene.
Metal and ceramic liners are often locked into the shell via a taper lock, meaning that the shell includes a taper and the liner includes a corresponding taper that fits into the taper of the shell. If properly seated, the shell taper and the liner taper engage one another and lock the liner into the shell. However, during insertion, the conical taper of the liner may become misaligned with the conical taper of the shell, thus preventing the intended surface-to-surface lock of the conical tapers. This is called cross-locking, characterized by a less stable edge or multi-point lock. If a misalignment exists during insertion, there is an increased risk of implant fracture and other complications during surgery.
When a liner becomes cross-locked, the surgeon must decide whether to leave the liner in the cross-locked position, try to remove the liner from the shell, or remove the entire implant construct. Leaving a cross-locked liner in the shell presents multiple risks to the patient including: increased wear, disassembly of the implant construct, non-optimal range of motion and implant fracture. Removing the liner from the shell or the entire construct also creates risks and adds complications to the surgery.
Therefore, there is a need for a liner that eliminates or greatly reduces the occurrence of a cross-locked taper junction.